The FOXO (Forkhead transcription factors, Class 0) proteins are a group of transcription factors involved in control of a variety of physiological, metabolic and developmental pathways. They are downstream effectors in a number of signaling pathways including insulin and growth factor signaling; they are also regulated by oxidative stress and nutrient deprivation. Cellular processes affected by FOXO activity include cell cycle control, differentiation, proliferation and apoptosis. Disregulation of FOXO mediated processes has been implicated in a number of pathologies including tumorigenesis, inflammation, diabetes and neurodegenerative conditions amongst others. Activity of FOXO transcription factors are controlled in part by their sub-cellular localization, in particular their localization to the nucleus from the cytosol, and their subsequent transcriptional activation.
Four FOXO proteins designated FOXO1, FOXO3a, FOXO4 and FOXO6 are present in human cells and their activity is controlled by a variety of mechanisms including stability (proteolytic cleavage), sub-cellular localization and transcriptional activation. Activity of the first three members of the family is controlled by cytosolic-nuclear translocation.
FOXO1 regulates expression of a number of genes that play critical roles in cell cycle and apoptosis. A pivotal regulatory mechanism of FOXO is reversible phosphorylation, catalyzed by kinases and phosphatases. Phosphorylation of FOXO1 is associated with 14-3-3 binding and cytosolic localization, whereas dephosphorylated FOXO1 translocates to the nucleus and is transcriptionally active.
Protein phosphatase 2A is one of the four major serine threonine phosphatases and is implicated in the negative control of cell growth and division. Protein phosphatase 2A holoenzymes are heterotrimeric proteins composed of a structural subunit A, a catalytic subunit C, and a regulatory subunit B. The PP2A heterotrimeric protein phosphatase is a ubiquitous and conserved phosphatase with broad substrate specificity and diverse cellular functions. Among the targets of PP2A are proteins of oncogenic signaling cascades, such as Raf, MEK, and AKT.
PP2A interacts directly with FOXO1 and dephosphorylates FOXO1. Inhibition of PP2A phosphatases rescues FOXO1-mediated cell death by regulating the level of the pro-apoptotic protein BIM. In addition, PP2A directly regulates FOXO3a subcellular localization and transcriptional activation. Without wishing to be held to any particular theory, it may be that the compounds described herein promote apoptosis by acting on FOXO transcription factors via activation of PP2A.
Myc proteins (c-myc, Mycn and Mycl) target proliferative and apoptotic pathways vital for progression in cancer and it is overexpressed and deregulated in many human cancers. The control of Myc abundance through protein degradation has attracted considerable interest and Ser-62 phosphorylation by a number of kinases has been shown to stabilize the protein. PP2A is responsible for Ser-62 dephosphorylation which primes the protein for ubiquitylation and degredation, thus PP2A functions as a negative regulator of Myc.
Prostate cancer is the second leading cause of cancer death in men in America, behind lung cancer. According to the American Cancer Society, approximately 1 man in 36 will die of prostate cancer. Male hormones, specifically testosterone, fuel the growth of prostate cancer. By reducing the amount and activity of testosterone, the growth of advanced prostate cancer is slowed. Endocrine therapy, known as androgen ablation, is the first line of treatment for metastatic prostate cancer. Androgen deprivation therapy for metastatic prostate cancer results in tumor regression and symptomatic improvement in the majority of patients. However, metastatic prostate cancer inevitably progresses despite castrate levels of serum testosterone. Several new therapies have been approved for patients with castration-resistant prostate cancer (CRPC); however, none are curative and tumors ultimately develop resistance. To combat CRPC new approaches and novel therapies are required.
Breast cancer can affect both men and women. Breast cancer is the most prevalent cancer in women, after skin cancers, with about 1 in every 8 women expected to develop invasive breast cancer at some point. One subset of breast cancer expresses the androgen receptor (AR), which has been implicated as a therapeutic target in that subset. About 10-20% of breast cancers—more than one out of every 10—are found to be triple-negative. “Triple negative breast cancer” refers to a breast cancer that does not contain estrogen receptors, progesterone receptors, or human epidermal growth factor receptor 2 (HER2). This means that the growth of the cancer is not supported by the hormones estrogen and progesterone, nor by the presence of too many HER2 receptors. Therefore, triple-negative breast cancer does not respond to hormonal therapy (such as tamoxifen or aromatase inhibitors) or therapies that target HER2 receptors, such as Herceptin (chemical name: trastuzumab). While these tumors are often treatable, the chemotherapy is not targeted, and response durations are short. For doctors and researchers, there is intense interest in finding new medications that can treat breast cancer.
The compounds described herein exhibit anti-proliferative effects and are useful as monotherapy in cancer treatment. Additionally, they can be used in combination with other drugs to restore sensitivity to chemotherapy where resistance has developed.